The present invention relates to a frame list cutting machine of the type having a cutter head for producing a V-shaped incision in a list member from one side thereof and thereby to effect severing of the list member, primarily for producing exactly mitered list pieces, said machine having an elongated supported table for supporting list members of various widths and provided with a V-shaped notch for receiving the cutter head by the cutting movement thereof, said cutter head being provided with two knife members arranged such that their cutting edges are located along the outer contour of the V-shaped knife member assembly, while the cutting edges, from the top point of the V-shaped knife structure, project along respective lines inclined relative the moving direction of the cutter head with a length such that the projection of the cutting edges onto the support table will at least roughly correspond to the shape and the length of the sides or edges of the V-notch of the support table.
Frame lists for picture frames may be cut in mitre by sawing, but for achieving corner joints of high quality the cuttings shall be absolutely clean, which is obtainable by means of a cutting machine of the above type. The list to be cut is placed on the elongate support table across the V-notch thereof and laterally against a guiding rail along the side of the support table at which the top point of the V-notch is located, and the cutter head is forced down for carrying out the V-incision in the list, normally by more operations, by which the cutter head is stepwise moved more and more inwardly over the list, such that the last cut, by which the list is cut over, will involve removal of a relatively thin end plate or slice portion from the list or rather from the respective ends of the two separate list lengths as separated by the over-cutting of the list. By this last operation the knives pass down along the edges of the V-notch in the support table, and it is essential, of course, that the cutting edges of the knives be located along the outer contour of the V-shaped knife system, i.e. the outsides of the knife plate members should project straightly upwardly from the cutting edges, that is in the upward moving direction of the cutter head, while the inclined cutting edge side should be located at the opposite, inner side of the knife plate member. The knives have to be rather heavily dimensioned, whereby they should also be rather thick, and by each cutting operation, therefore, they will produce a considerable displacement of the material being cut. A resulting consequence is the said desire of cutting off, by the last cutting operation, only a narrow slice portion of the list material in order to obtain an entirely clean final cutting of the resulting list ends.
The two knives have to meet in the top point of the V-structure such that the respective ends of the cutting edges thereof are brought entirely together in this point. It is a resulting requirement that the interengaging end surfaces of the knife plate members should be correspondingly oblique to enable such a meeting of the knife members in the said top point area.
Moreover, the two knife members are normally arranged in mutual angular relationship not only corresponding to the V-notch in the support table, but also in the direction normal to the support table, i.e. in the moving direction of the cutter head, such that the knives, from their said top point, are inclined both rearwardly and downwardly. Hereby the cutting into the lists will take place in a progressive manner, which is a practical necessity. It is hereby usual advantageous that the adjacent ends of the knife plates abut each other also above the top point of the cutting edges, and in this abutment area the end faces of the knife plates should thus be oblique relative the knives. Here the knife ends should be cut obtuse-angled relative the cutting edge, while the same end surfaces, as mentioned above should be acute-angled in the cross direction.
Thus, the adjacent knife ends have to be machines in a special and accurate manner, while the outer knife ends can be left with any simple shape.
The knives, which are rather expensive elements, will be worn in use, but they can be demounted for sharpening, and already for this reason it is important that the knives meet each other also above the top point of the cutting edges, because these edges will then meet each other again when the knives, after the sharpening, are remounted in unchanged positions.
The knives are totally symmetrical about their meeting plane at the said top point, but they cannot replace each other, i.e. they are present as a set of two different knives.
On this background the invention takes its starting point in the finding that the knives, in practice, are worn most on their respective halves adjacent the top point of the knife structure, i.e. more than on their outer halves. This is mainly due to the fact that ordinary list cutting machines are adapted to handle lists of different widths, up to a relatively large width, while in practice the vast majority of lists actually handled are relatively narrow, i.e. the outer half of the knives are only seldom in operation. On this background it has been realized that it would be advantageous if the knives were turnable to make the outer halves operative after some time, but the required basic shape of each knife simply forbids such a turning, which would imply an operatively impossible location of the inclined side of the cutting edge at the outside of the V-knife structure.
The invention is based on the consideration that the knives are in fact turnable into new, fully operational positions, provided they are hereby additionally interchanged and mounted with their formerly outer ends meeting at the top area of the reproduced V-knife structure. According to the invention, for enabling such a remounting of the knives the list cutting machine is characterized in that each of the two knives are shaped symmetrically about a central cross plane in such a manner that the knives are interchangeable on the cutter head to form a reestablished V-shaped knife member assembly with inverted length orientation of each of the knives.
When the knives are thus similarly shaped at both ends each knife will be mountable on either side of the cutter head, and the knives will be interchangeable, therefore, when they are additionally turned in such a manner that their formerly outer ends are now brought into the inner area of the V-knife structure. Here they will be exposed to an increased wear, as explained, while the formerly inner halves of the knives will be exposed to a reduced wear, such that the lifetime of the knives will be noticeably prolonged.
Another and important consequence of the invention is that the two knives of the knife set may now be entirely similar, whereby the extra machining work required for the shaping of the "outer" end of the knives, will already to some degree be compensated for by the fact that it will no longer be necessary to distinguish between knives for the respective left and right sides of the cutter head.
The invention also comprises the said knives as provided with symmetrically shaped opposite end portions. Obviously such a knife will be more expensive than the previously known knives, of which only one end has been finely machined, but the extra price is more than compensated for by the fact that the knife or a set of such knives will be turnable for the achievement of an almost doubled durability under normal conditions of use.
In the following the invention is described in more detail with reference to the drawing, in which: